The basic characteristics of electronically commutated reluctance motors operated as continuously rotating variable-speed drives are well known, since they are members of the class of variable reluctance motors, commonly used in stepper motor applications. As variable-speed drives, electronically commutated reluctance motors are designed for efficient power conversion rather than for particular torque or control characteristics typically required in stepper motor applications, and the pole geometry and control strategies differ accordingly. For example, the number of rotor poles is relatively small in an electronically commutated reluctance motor (cf., variable reluctance stepper motors), giving a large step angle, and the conduction angle is, generally, modulated as a function of both speed and torque to optimize operation as a variable-speed drive. As a term of art, these variable-speed reluctance motors are generally known as switched reluctance.
Because of recent developments in power semiconductor devices such as power MOSFETs and insulated gate thyristors (IGTs). Switched reluctance motors have gained attention relative to other types of motors suitable for variable-speed drive applications. This increased attention derives from the fact that switched reluctance motors compare very favorably with other types of motors typically used as variable-speed drives. For example, the speed versus average torque curves for switched reluctance motors are very similar to the curves for brushless permanent magnet (PM) motors--e.g., the curves are fairly linear with no discontinuities of torque. Additionally, switched reluctance motors are the cheapest type of motor to manufacture. They are rugged and robust and therefore well suited for heavy duty use. They have excellent heat dissipation qualities, and they do not require brushes or slip rings. The drive circuits for switched reluctance motors are the simplest and lowest cost compared to drives for other motors. Moreover, using state-of-the-art semiconductor technology for controllers, the efficiency of switched reluctance motors compare very favorably with other classes of variable-speed motors such as inverter-driven AC motor and PM motors.
Although the foregoing comparative features are favorable, switched reluctance motors are also known to have several disadvantages which are common to all variable-speed drive motors. Specifically, copper, hysteresis and eddy current losses limit motor efficiency, especially at relatively high RPMs.